This invention generally relates to an optical projection system, and specifically relates to an optical projection system for photolithography used in producing integrated circuits, large-scale integrated circuits, or other semiconductor devices.
During the fabrication of semiconductor devices such as integrated circuits, photolithography transfers an image from a photographic mask to a resultant pattern on a semiconductor wafer. Such photolithography generally includes a light exposure process in which a semiconductor wafer is exposed to light having information of a mask pattern. Optical projection systems are used to perform the light exposure process.
In general, transferred mask patterns are very fine so that optical projection systems are required to have a high resolution. The high resolution necessitates a large numerical aperture of the optical projection system and also an essentially null aberration of the optical projection system in the light exposure field. In addition, since a plurality of light exposure steps are sequentially performed to transfer different mask patterns onto a semiconductor wafer in an overlappling manner to complete a composite mask pattern, the optical projection system is required to have a very small distortion.
Japanese published examined patent application 57-12966 discloses an optical projection system using a g-line and i-line light source. This prior-art optical projection system generally requires a large number of thick lenses.
Japanese published unexamined patent application 60-28613 discloses an optical projection system using an excimer laser as a light source. This prior-art optical projection system generally requires a large number of lenses. Furthermore, in this prior-art optical projection system, when an ArF excimer laser is used, the laser light is absorbed by lenses of the system at a high rate.